Most computer systems include a data storage device comprising of a rotating magnetic coated disk and a transducer for reading and writing information stored on the magnetic material of the disk. Such systems are normally characterized by storage density, access speed to data locations, reliability, and data integrity. One of the principal parameters which significantly affects the system characteristics is the position of the magnetic transducing head relative to the rotating disk. The relative air flow between the disk rotating at a high rate and a head biased toward the disk causes the head to fly on an established cushion of air. Generally, the smaller is the head-to-disk spacing the higher is accuracy of transduction of information stored on the disk. The head-to-disk spacing referred to as the "head gap" or a "flying height" for conventional high performance systems is on the order of several tens of nanometers. Flying aerodynamics vary for different heads, disks, rotation speeds resulting in different flying heights and head orientations. Therefore, in the design process, as well as in production, it is important to provide precise control of the flying height and orientation of the head to meet desired performance criteria.
At the present time, several optical techniques are used to measure a nanometric gap between a magnetic head and a rotating magnetic disk.
One measuring method is based on optical interferometry. This method uses a mutual interference effect wherein two optical beams produce lines, bands, or fringes which are either alternately light and dark or variously colored. In order to measure a gap between two objects having nearly parallel mutually facing surfaces, where one of the objects is transparent, a beam of light is directed into the gap to be measured through the body of the transparent object in such a way that the axis of the beam is essentially normal to the facing surfaces. Beams reflected from the surfaces of both objects are superimposed at a detector element and the interference fringes are read. It is known from optics that the detected light intensity depends on the ratio of the path difference between two beams to the radiation wavelength. This relationship is used as a calibration table for gap measurements as the path difference between the beams is twice the gap.
A particular application of optical interferometry for measuring the nanometric gap between a magnetic head and a flat reference disk made of an optically transparent material such as glass is disclosed in U.S. Pat. No. 4,813,782 issued in 1989 to Yagi et al. In the apparatus described in this patent, the operating conditions of a hard disk drive are simulated by rotating a reference disk with a high speed, and a magnetic head to be tested is biased toward the reference disk, e.g., by a spring, and flies above the disk on a dense air cushion. As a disk is rotating, a light beam is directed through the transparent reference disk from the side opposite to the magnetic head. The beams reflected from the surface of the disk and the surface of the head interfere with each other producing interference fringes. These fringes are detected and analyzed for determining the gap between the magnetic head and the reference disk using a calibration curve.
The main drawback of the above method is inaccuracy of the calibration curve near its maximum and minimum points, where measurement accuracy is significantly low due to small changes in the signal with the variation of the gap (so called "flat regions" of the calibration curve). This problem is especially pronounced in systems based on the above principle and intended for measuring head gaps that are much less than one quarter of the optical wavelength. Moreover, commercially available devices are unable to take measurements at several points on the magnetic head at the same time. Therefore, time consuming point-by-point measurements have to be performed in order to obtain a map of surface-to surface proximity.
Another optical method that is used to measure the gap between objects is based on a phenomenon known as "frustrated" total internal reflection. Total internal reflection is observed when electromagnetic radiation (e.g., a light beam) is incident on an interface between two media at an oblique incidence angle. If the radiation propagates from the side of the optically denser of the two media, e.g., the medium having the higher index of refraction of the two referred to below as the first medium, and the incidence angle exceeds (as measured from the propagation axis) a certain critical value that depends on the ratio of the refractive indexes of the two media, all radiation is reflected back to the first medium and none enters the other, or second medium, and the reflection is "total".
However, if the second medium is a thin film, followed by a third medium, which has a higher refractive index than the first medium, a part of the incident radiation is reflected back into the first medium, but a part propagates into the third medium. In other words, the internal reflection is not total and therefore is called a frustrated total internal reflection even though the angle of the incident radiation and the indices of refraction of the first and second media would appear to be appropriate for total internal reflection. In this case of the frustrated total internal reflection, a fraction of radiation reflected back into the first medium depends on a ratio of the thickness of the second medium to the radiation wavelength, a complex refractive index of the third medium, and polarization of the incident radiation. Such systems are more sensitive to the variation of nanometric gaps and therefore are suitable for measuring the "flying height" between a magnetic head above a reference disk with higher accuracy than the apparatuses based on the principle of optical interference.
An apparatus which determines the proximity of a stationary glass surface to another surface using the phenomenon of frustration of total internal reflection is disclosed in U.S. Pat. No. 4,681,451 issued to Guerra et al in 1987. In the apparatus, a glass block is used to substitute a conventional magnetic head. Its spacing from a magnetic disk is then imaged by a video camera detecting intensity distribution of the light reflected back into the glass. The magnetic disk may be rotated to simulate aerodynamic characteristics.
The main disadvantage of this proximity imaging device is its inability to test dynamic behavior and to measure the flying height of an actual magnetic head, as may be needed by a magnetic head manufacturer, or a consumer, for quality control purposes. Even though some of the conditions inside a disk drive can be simulated by executing a replica of the head in glass, the results obtained in this manner are inaccurate. Furthermore, because the size and mass of the optical system required is substantial, the device can not be used to test miniature flying magnetic heads, nor can it exhibit the dynamics of an actual spring mounted head weighing a small fraction of a gram. Thus, the apparatus cannot be used to test the characteristics of an a actual head.
A different apparatus which determines the proximity of a rotating glass surface to another surface using frustrated total internal reflection is disclosed in U.S. Pat. No. 5,257,093 (issued to Mager et al in 1993). In that patent, a device is used to determine the gap between a real magnetic head and a surrogate magnetic disk, represented by a pair of glass lenses. One of the glass lenses may be set into motion to develop aerodynamic characteristics establishing the spacing between the surface of the glass lens and the magnetic head close to the actual device. The stationary second lens with two prisms is used to couple illumination energy into the surface undergoing frustration of the total internal reflection and to view and measure resulting internal reflection for purposes of determining the distance to the head.
Two lenses and two prisms required by this apparatus are physically large and heavy. The apparatus needs complicated alignment of prisms mounted to one of the surfaces. In order to withstand relative motion at several thousand revolutions per minute, these lenses must be fabricated to severe tolerances and must be placed in a strong housing in case they are broken while rotating. Furthermore, the rotating lens is subject to rapid deterioration and, therefore, requires frequent replacement. The replacement is followed by the procedure of full and complicated alignment. Thus, such system is costly, complicated and has a limited scope of application.
The above problem is solved in an apparatus described in a pending U.S. patent application Ser. No. 08/476,626, now U.S. Pat. No. 5,677,805 of the same applicant, incorporated herein by reference. The apparatus described in that application utilizes an extremely simple single flat reference disk made of a transparent material such as glass. Light enters the disk from one side of the disk at an angle to the flat surface higher than the critical angle of the total internal reflection and propagates through the glass. When a magnetic head approaches the flat surface of the disk, frustration of the total internal reflection takes place. It is also known from the field of optics that the phenomenon of the frustrated total internal reflection is always accompanied by so-called photon-tunneling effect. This effect consists in ability of light to penetrate from a first medium to a third medium through a thin a thin second medium. The intensity of the light penetrated through the thin second medium, which in the case under consideration is a gap between the magnetic head and the reference disk, is complementary to the intensity of light reflected back into the reference disk in the case of the aforementioned frustrated total internal reflection.
In the apparatus of U.S. patent application Ser. No. 08/476,626, the proximity of the magnetic head to the disk is measured as intensity of the light that left the disk due to photon tunneling and was scattered by the surface of the magnetic head. As the disk is transparent, the scattered light is measured by a detector located on the side of the disk opposite to the head.
Although this measuring system is extremely simple and inexpensive, it produces a rather weak signal which is difficult to detect on a background of the noise. Such systems are suitable for testing small-batch production of magnetic heads, i.e., for conditions where the manufacture of more sensitive and accurate measurement systems may appear to be economically unjustified.